1. Field of the Invention
The present invention relates to a sound detecting mechanism comprising a pair of electrodes forming a capacitor on a substrate, in which one of the electrodes is a back electrode forming perforations therein corresponding to acoustic holes and the other of the electrodes is a diaphragm.
2. Description of Related Art
Conventionally, condenser microphones are frequently used in mobile phones, for example. A typical construction of condenser microphones is shown in FIG. 6. This condenser microphone comprises a metal capsule 100 including a plurality of perforations “h” corresponding to acoustic holes formed therein, a fixed electrode 300 and a diaphragm 500 provided inside the capsule to be opposed to each other with a spacer 400 therebetween to maintain a predetermined gap, a substrate 600 fixed and fitted to a rear opening of the capsule 100, and an impedance converting element 700 made of J-FET or the like and mounted to the substrate 600. With this type of condenser microphone, a high voltage is applied to a dielectric material formed on the fixed electrode 300 or the diaphragm 500 to be heated to generate electric polarization and produce an electret membrane allowing a residual electric charge to remain on a surface thereof (an electret membrane 510 is formed in a diaphragm body 520 made of metal or conductive film which constitutes the diaphragm 500 in FIG. 6), thereby to provide a construction that requires no bias voltage. When the diaphragm 500 is vibrated by sound pressure signals of a sound, a distance between the diaphragm 500 and the fixed electrode 300 is changed to vary capacitance. The variation of capacitance is outputted through the impedance converting element 700.
Another conventional sound detecting mechanism has the following construction. This sound detecting mechanism comprises a substrate (110) constituting a diaphragm and a substrate (108) constituting a back face plate (103) (corresponding to the back electrode of the present invention), both substrates being superimposed through an adhesive layer (109) and then adhered to each other through heat treatment. Then, the substrate (108) acting as the back face plate is ground to obtain a desired thickness. After an etching mask (112) is formed on each of the substrates (108) and (109), the substrates are treated with an alkali etching liquid thereby to obtain the diaphragm (101) and the back face plate (103). Next, the back face plate (103) is reticulated (corresponding to the perforations of the present invention). An insulating layer (111) is etched with hydrofluoric acid, with the back face plate (103) acting as an etching mask, thereby to form a void layer (104) (see Patent Document 1, for example: the reference numbers are quoted from the cited document.)
Patent Document 1: Japanese Patent Publication No. 2002-27595 (paragraph [0030] through [0035], FIG. 1 and FIG. 3).
In order to increase output (improve sensitivity) of the conventional microphone shown in FIG. 6, it is required to increase the capacitance between the fixed electrode 300 and the diaphragm 500. In order to increase the capacitance, an area of superimposition of the fixed electrode 300 and the diaphragm 500 should be increased. Alternatively, it will be effective to reduce the gap between the fixed electrode 300 and the diaphragm 500. However, an increase in the area of superimposition of the fixed electrode 300 and the diaphragm 500 would lead to an enlargement of the microphone per se. On the other hand, in the construction having the spacer 400 noted above, there is a limitation in reducing the distance between the fixed electrode 300 and the diaphragm 500.
Also, the electret condenser microphones often utilize a high polymeric organic substance such as FEP (Fluoro Ethylene Propylene) or the like in order to produce a permanent electric polarization. The microphone using such a high polymeric organic substance has poor heat resistance, and thus is hardly capable of enduring the heat in time of re-flow treatment when mounted on a printed board, for example. The microphone, therefore, cannot be given re-flow treatment when mounted on the printed board or the like.
In view of the above, as described in Patent Document 1, it is conceivable to employ a construction including a back electrode and a diaphragm formed on a silicon substrate by micro fabrication technique. A sound detecting mechanism having such a construction is compact and yet is capable of enhancing sensitivity by reducing the distance between the back electrode and the diaphragm. Further, the mechanism can undergo re-flow treatment while requiring a bias supply. However, according to the technique set forth in Patent Document 1, the diaphragm is formed by etching a monocrystal silicon substrate with an alkali etching liquid, which makes it difficult to control the thickness of the diaphragm. As a result, it is difficult to obtain a required thickness for the diaphragm.
In considering control of the thickness of the diaphragm here, it is effective to utilize a single crystal silicon on insulator (SOI) wafer to improve the controllability of the thickness of the diaphragm in the process of forming the diaphragm by etching the silicon substrate with the alkali etching liquid. More particularly, according to this method, a built-in oxide film of the SOI wafer can be utilized as a stop layer for etching with the alkali etching liquid, thereby to control the thickness of the diaphragm by selecting the thickness of an active layer of the SOI wafer.
As a different method to the above, it is conceivable to utilize, instead of using the SOI wafer, an SOI structure wafer in which silicon oxide film or silicon nitride film is formed on the monocrystal silicon substrate as an etching stop layer to function as a stop layer in time of etching with the alkali etching liquid, and polycrystal silicon is formed on the etching stop layer. With this SOI structure wafer, it becomes possible to stop etching with the etching stop layer when the silicon substrate is etched with the alkali etching liquid, thereby to enhance the controllability of the thickness of the diaphragm.
However, in the method utilizing the SOI wafer or the method utilizing the SOI structure wafer, the sound detecting mechanism has a construction with a plurality of materials (films or layers) laminated on monocrystal silicon acting as a base. Thus, while a relatively thin diaphragm can be formed with high accuracy by stopping etching with the etching stop layer to form the diaphragm, the diaphragm is distorted by an inner stress caused by a difference between coefficients of thermal expansion of the plurality of materials laminated on the monocrystal silicon, which might lead to a disadvantage of deteriorating the vibration characteristic to hamper vibrations faithful to sound pressure signals when the diaphragm contacts the back electrode or even when the diaphragm does not contact the back electrode.